Porous, filamentary mats of thermoplastic material have been found to be useful for various purposes. For examples, porous, filamentary mats have been used as erosion-controlling mats. However, at least one porous, filamentary mat, which has been marketed for use as an erosion-controlling mat, has had an unduly high profile and has had insufficient resistance to excessive elongation in the lateral direction. Another porous, filamentary mat, which has been marketed for use as an erosion-controlling mat, has been so stiff that it could not conform to the contours of the soil which it was supposed to protect. As a result, prior porous, filamentary mats, which have been marketed as erosion-control mats, have not been satisfactory.